Do sodium and potassium forms of Na,K-ATPase differ in their secondary structure?

Infrared spectroscopy in the amide I region of purified membrane-bound Na,K-ATPase preparation shows that Na+- and K+-bound forms of the enzyme have almost the same secondary structure. No difference is detected in the beta-structure (pleated sheets) content. This is contrary to the statement of the recent paper (Gresalfi, T. J., and Wallace, B. A. (1984) J. Biol. Chem. 259, 2622-2628) where a similar preparation was examined by circular dichroism spectroscopy and it was claimed that net 7% of protein peptide groups undergo a beta-sheet to alpha-helix conformational change upon Na,K-ATPase conversion from the K+ to the Na+ form. The discrepancy of the results is most likely caused by the particulate nature of the enzyme preparations used that could lead to optical artifacts in CD but not in IR measurements. A thorough comparison of IR spectra of these enzyme forms has revealed a very minor spectral difference which could suggest conformational perturbations, if any, of a much lower scale and another type than that claimed by Gresalfi and Wallace. The K+ form tends to absorb slightly more in the region of the alpha-helix band. This could reflect some distortion or a transition to a random coil structure of a small fraction of alpha-helical segments (less than or equal to 2% protein peptide groups) upon the enzyme conversion from the K+ to the Na+ form.